Pulp sludge ashes generated by the incineration of pulp sludge in a papermaking process are used as cement additives, etc., because of hydraulic properties (i.e., pozzolanic properties). However, the majority of these pulp sludge ashes are buried in landfills, since methods capable of increasing their added value are insufficient in view of the amount of their generation.
Acidic wastewater generated in the production of copper, zinc or titanium or the production of sodium hydroxide or chlorine by the electrolysis of seawater or salt-dissolved water (hereinafter, referred to as saline water) is neutralized with calcium hydroxide to form sludge. The sludge is dehydrated to a water content of 60-80% to form so-called “waste gypsum”. This reaction is illustrated by the following equation:H2SO4+Ca(OH)2→CaSO4+2H2O
The waste gypsum has a chemical molecular formula of CaSO4 which is the same as that of general gypsum. However, the waste gypsum has different chemical properties from general gypsum, so that it has no hydraulic properties even when it is not only in a wet state but also dehydrated by drying. Particularly, at a water content of 60-80%, the gypsum cannot provide a hardened material having strength sufficient to make handling easy, even when it is mixed with a cement having special composition. Thus, it was difficult to provide a cost-effective composition using the waste gypsum.
Although the waste gypsum is generated in various industrial fields at very large amounts, there are no methods of treating harmful components contained in the waste gypsum and recycling the waste gypsum in an economic manner. For this reason, it is buried in the underground at many costs, thus increasing the expense of manufacturers for wastewater disposal and causing environmental contamination.